The current standard technique to transfer particles or cells from one solution to another at the macroscale level involves centrifugation and re-suspension. This is a manual labor and time intensive process that is not easily miniaturized or integrated due to the bulk of the centrifuge machine and manual pipetting steps required. Centrifugation and pipetting steps are, of course, labor and time intensive processing steps. Attempts have been made to miniaturize this capability using microstructures to divert cells while not diverting the fluid component. For example, Morton et al. discloses an asymmetric post array used in pressure-driven microfluidic flow to move particles of interest across multiple, independent chemical streams. See Morton, K. J. et al., Crossing microfluidic streamlines to lyse, label and wash cells, Lab Chip 8, 1448-1453 (2008).
Others have used dielectrophoresis (DEP) to transfer particles electrically. For example, Tronay et al. have used activated DEP electrodes in a microfluidic device where particles can be continuously functionalized in flow. The device uses a particle exchanger which allows for particles to be taken from one medium and exposed to some reagent while minimizing mixing of the two liquids. In the exchanger, two liquids are brought in contact and particles are pushed from one to the other by the application of a dielectrophoretic force. See Tornay, R. et al., Dielectrophoresis-based particle exchanger for the manipulation and surface functionalization of particles, Lab Chip 8, 267-273 (2008). Still others have used acoustic manipulation of suspended particles, in which particles in a laminar flow microchannel are continuously translated from one medium to another with virtually no mixing. See Petersson, F. et al., Carrier Medium Exchange through Ultrasonic Particle Switching in Microfluidic Channels, Anal. Chem. 77, 1216-1221 (2005). Yet another approach uses hydrodynamic filtration in which the virtual width of flow in a microchannel determines the size of filtered cells/particles. See Yamada M. et al., Millisecond treatment of cells using microfluidic devices via two-step carrier medium exchange, Lab Chip, 8, 772-778 (2008).
While some microfluidic-based sorting devices have been proposed for solution exchange, there are concerns about device complexity, and the speed of operation. In many cases, the speed of exchange is rather slow and cannot be integrated, for example, with additional downstream processing applications such as cytometry. Microfluidic-based solution exchange systems should have high-throughput, be easy to multiplex, should be able to position particles or cells for possible downstream interrogation, and should have fast transfer. The prior techniques do not satisfy all of these criteria.